1. Field of the Invention
The present invention relates to an electronic inductance circuit suitable for use with a current supply circuit for supplying speech (or channel) current to a subscriber terminal of a subscriber circuit.
2. Description of Related Art
Subscriber circuits for electronic switching apparatus are formed with many circuit elements mounted on a semiconductor substrate according to demands such as smaller size and lower power consumption. Electronic inductance circuits are used for inductance components which are an essential structural element of subscriber circuits. When an electronic inductance circuit is used, coils become unnecessary, so smaller size can be realized. An example of a conventional electronic inductance circuit is disclosed in the reference "SHINGAKU GIHOU SE79-107, pp. 79-86".
FIG. 13 is a circuit diagram showing an equivalent circuit of the electronic inductance circuit noted in the above-mentioned reference. This electronic inductance circuit is used in a speech current supply circuit within a subscriber circuit, and has a dual circuit structure. When this electronic inductance circuit is used as a single circuit structure, the electronic inductance circuit can be represented as shown in FIG. 14. Here, we will omit an explanation of the circuit itself shown in FIG. 13.
The electronic inductance circuit 10 shown in FIG. 14 comprises an NPN transistor Tr1, resistors 12, 14, and 16, and capacitor 18. FIG. 14 also shows terminal A1 as an input terminal and terminal A0 as an output terminal. Between these two terminals A1 and A0, the main current path between the collector terminal and the emitter terminal of NPN transistor Tr1 and the resistor 16 are connected in series from the terminal A1 side. Also, between the two terminals A1 and A0, the resistors 14 and 12 are connected in series from the terminal A1 side. These resistors 14 and 12 are provided in parallel in relation to the transistor Tr1 and the resistor 16. Also, a connection point 20 between the resistors 14 and 12 is connected to the base terminal of the transistor Tr1. Furthermore, the capacitor 18 is connected in parallel to the resistor 12.
This type of electronic inductance circuit 10 for speech current supply circuits allows direct current to pass on the one hand, while also suppressing alternating current components.
First, we will explain the operation of allowing direct current, or speech current, to pass. A speech current IL flows within the electronic inductance circuit 10 in the sequence of the terminal A1,the main current path of the NPN transistor Tr1, the resistor 16, and the terminal A0. Here, a terminal A1 direct current operating point (specifically, a voltage drop of V10 between the terminals A1 and A0) is needed. This voltage drop V10 is the sum of the voltage drop at the resistor 14, the voltage drop VBE between the base terminal and the emitter terminal of the transistor Tr1, and the voltage drop at the resistor 16, as represented by equation (1). Note that the current amplification factor hfe of the transistor Tr1 is sufficiently high, so the base current can be ignored, and a current of equal size flows to both the resistors 14 and 12. EQU V10=R1.times.(R2.times.IL+VBE)/R0+R2.times.IL+VBE (1)
Signals R0, R1, and R2 represent the resistance values of the resistors 12, 14, and 16, respectively.
Next, we will explain the alternating current impedance of electronic inductance circuit 10. Generally, the resistance values R1 and R0 of the resistors 14 and 12 which are provided in a location that is not in the path of speech current IL are sufficiently large in comparison to the resistance value R2 of the resistor 16. In fact, the current amplification factor hfe of the transistor Tr1 is sufficiently high, so it is possible to ignore the alternating current that flows to the resistor 14. Therefore, the alternating current impedance R10 between the terminals A1 and A0 is represented by equation (2). EQU R10=j.omega.{(1/gm+R2) .times.C1.times.R1}+(1/gm+R2).times.(R1/R0+1)(2)
Furthermore, the signal C1 and gm represent the capacity of the capacitor 18 and the conductance of the transistor Tr1, respectively. Also, ##EQU1## and the signal .omega. represents the alternating current angular frequency.
When electronic inductance circuit 10 allows direct current to pass, it is preferable to have the voltage drop V10 represented by equation (1) be small. On the other hand, to have alternating current components suppressed, each parameter value is determined such that the alternating current impedance R10 represented by equation (2) will be suitable.
Furthermore, the electronic inductance circuit 10 shown in FIG. 14 uses an NPN transistor, but it is also acceptable to construct the circuit using a PNP transistor. FIG. 15 is a circuit diagram showing the structure of a conventional electronic inductance circuit constructed using a PNP transistor.
The electronic inductance circuit 10a shown in FIG. 15 comprises a PNP transistor Tr2 resistors 12, 14, and 6, and a capacitor 18. Also, terminal A1 is shown as an input terminal and terminal A0 is shown as an output terminal in FIG. 15. Between these two terminals A1 and A0, a main current path between the collector terminal and the emitter terminal of the PNP transistor Tr2 and the resistor 16 are connected in series from the terminal A0 side. Also, between the terminals A1 and A0, the resistors 14 and 12 are connected in series from the terminal A0 side. These resistors 14 and 12 are provided in parallel in relation to the transistor Tr2 and the resistor 16. Also, the connection point 20 between these resistors 14 and 12 is connected to the base terminal of the transistor Tr2. Furthermore, the capacitor 18 is connected in parallel to the resistor 12.
In this way, the circuit shown in FIG. 15 is symmetrical with the circuit shown in FIG. 14. Even with the circuit shown in FIG. 15, the voltage drop and alternating current impedance between the terminals A1 and A0 are represented by the equations (1) and (2) described above, respectively.
However, the electronic inductance circuit 10 (10a) shown in FIGS. 14 and 15 has the problems noted below.
(a) The voltage drop V10 between the terminals A1 and A0 of the electronic inductance circuit 10 (10a) is represented by the above equation (1). The voltage drop V10 is affected by fluctuations in the current value of the direct current IL and by fluctuations in the voltage drop VBE between the base terminal and the emitter terminal of the transistor Tr1 (Tr2). This voltage drop VBE between the base terminal and the emitter terminal fluctuates according to temperature fluctuations. Therefore, to correctly operate the electronic inductance circuit 10 and to avoid distortion of the output signal of the transistor Tr1 in relation to the input alternating current signal, it is necessary to take into consideration fluctuations in direct current and fluctuations in temperature. To do this, the voltage drop between the terminals A1 and A0 must be made sufficiently large. However, this means the power consumption of the electronic inductance circuit 10 becomes large.
(b) When the electronic inductance circuit 10 is used for a subscriber circuit, there are times when one does not wish to implement the alternating current component suppression function. In this case, as shown in FIG. 16, a switch SW1 is connected in parallel to the electronic inductance circuit 10. When one does not wish to implement the alternating current component suppression function, this switch SW1 is connected. In this way, since it is necessary to provide the switch SW1, the number of parts to be used in the conventional electronic inductance circuit increases, and the circuit scale thereof increases as well.
(c) When the electronic inductance circuit 10 is used for a subscriber circuit, the alternating current signals input to the electronic inductance circuit 10 have their various amplitudes such as howler tones and busy-back tones. Then, the amplitude of the output signals of the electronic inductance circuit 10 differ for each type of these input signals. In this kind of use environment, depending on the type of input signal, it may be necessary to change the operating range of the electronic inductance circuit 10 (the largest alternating current amplitude that can be handled). To do this, the resistance value R0 of the resistor 12 shown in FIG. 14 must be changed. However, when the electronic inductance circuit 10 is fabricated on a semiconductor substrate, it is difficult to use a variable resistor for the resistor 12.
(d) Also, there are cases when an alternating current signal is input to a circuit using the electronic inductance circuit 10, and this alternating current signal is superimposed on a signal based on the direct current IL. As with the conventional example shown in FIG. 17, the alternating current signal generated at the alternating current signal generating source 22 is input to the electronic inductance circuit 10 via an alternating current signal superimposing circuit 24. Because the alternating current signals are superimposed in this way, there is an increase in the number of parts to be used in the conventional electronic inductance circuit and circuit scale thereof.
(e) Also, when the frequency of the alternating current signal superimposed in the electronic inductance circuit 10 becomes high, it becomes impossible for the transistor Tr1 that constructs the electronic inductance circuit 10 to keep up with this, and the circuit alternating current impedance decreases. When the alternating current impedance is low, there is an effect on the impedance of the end terminal of the subscriber circuit.
(f) Also, when the electronic inductance circuit 10 is used for a subscriber circuit, there are times when a user may wish to monitor the supply current to a subscriber terminal. In this case, as shown in FIG. 18, a resistor RE is connected in series to the electronic inductance circuit 10, and a current monitor circuit 26 is connected in parallel to this resistor RE. Thus, the number of parts to be used in the conventional electronic inductance circuit increases, and the power consumption of the circuit increases due to a voltage drop at the resistor RE.
(g) Also, when the electronic inductance circuit 10 is used for a subscriber circuit, it becomes necessary to block the speech current IL. To do this, as shown in FIG. 19, a cutoff switch SW2 is connected in series to the electronic inductance circuit 10. However, this causes the parts count and circuit scale to increase. Furthermore, when the switch SW2 is composed from a thyristor or transistor, a voltage drop occurs in that part, and power consumption increases.